There is a demand for a technique that visualizes a distribution of a specific molecule in an organism from a cell level to an individual level. For example, a lifetime of a free radical in an organism is short, and a spatial distribution thereof is desired to be measured during a short period of time (for example, several seconds). As an apparatus for detecting a free radical, there is an electron spin resonance (ESR) imaging apparatus. The ESR imaging apparatus can selectively detect only a free radical by observing a magnetic moment of an unpaired electron spin of a free radical using resonance absorption of electromagnetic waves.
In a general ESR imaging method, a distribution of a kind of free radical can be visualized, but a distribution of two or more kinds of free radical cannot be visualized in a single measurement. As techniques for visualizing a distribution of two or more kinds of free radical, three techniques below are reported.
As a first technique, an example is reported in which a distribution of 14N nitroxy radical and a distribution of 15N nitroxy radical are simultaneously visualized using an overhauser effect MRI (see Patent Literature 1 and Non-Patent Literature 1). In the overhauser effect MRI, electron spins in two radicals are separately excited using different magnetic fields by using a fact that an ESR absorption spectrum of 15N nitroxy radical is different from an absorption spectrum of 14N nitroxy radical when an electron spin is excited. Thus, a distribution of two kinds of free radical is successfully visualized.
As a second technique, an example is reported in which a distribution of two or more kinds of free radical is visualized by estimating a spatial distribution of an electron spin using iterative deconvolution (see Non-Patent Literature 2). In this example, a calculation is performed to reproduce measured projection data using two kinds of spectra. For this reason, time for a calculation of projection data and an image reconstruction is required, and there is a limit to short time processing.
As a third technique, an example is reported in which a distribution of multiple kinds of free radical is visualized by a spectral-spatial ESR imaging method (see Non-Patent Literature 3). The spectral-spatial ESR imaging method can perform a measurement without limiting a kind of free radical. However, to visualize a two-dimensional distribution of a free radical, a three-dimensional spectral-spatial imaging is needed to be performed. To perform the three-dimensional spatial imaging, generally, a spectrum of about the cube of the number of projections (Np) of each direction is needed to be obtained while changing a magnitude and a direction of a magnetic field gradient, and a measurement time increases.